>Computers, which process data vary fast, are good.
>Computers that process data very fast are good.
Where did you get these sentences from? Personally, I don't think they are very good examples of the difference between 'that' and 'which'.
'that' is restrictive and the second sentence is correct. You are limiting the computers to only the fast ones.
I'm not so sure you can say "computers, which process data very fast, are good." Which computers?? You need "that" in that sentence, without a comma.